POSTORE BASITE DYKES OF THE VORONTSOVKA GOLD DEPOSIT (NORTHERN URALS, RUSSIA) – EARLY CARBONIFEROUS IMPULSE OF POST-COLLISIONAL MAGMATISM

Two dyke complexes, pre-ore and post-ore, were identified for the large Vorontsovka gold deposit of finely interspersed gold (Carlin type), and age estimates for the latter were obtained. The gold deposit is located in the eastern part of the Tagil zone of the Urals and is localized in the southwestern exocontact part of the Auerbach gabbro-diorite-granodiorite intrusive. The ore field contains close-arranged bundles of Devonian pre-ore and Carboniferous post-ore mafic dykes, ranging from picrobasalts and gabbro to quartz diorites and monzonites. The enrichment of high–ion lithophilic elements (Cs, Rb, Ba, Pb, Sr) with relatively high-charge and rare-earth elements, Ta-Nb-minimum and Pb-maximum indicate the formation of dykes of the Vorontsovka deposit in a suprasubduction environment in a sufficiently thick continental crust. The late dyke group mainly includes high-potassium mafic rocks somewhat enriched in phosphorus (K₂O > 2 wt.%, P₂O₅ up to ~1 wt. %); they correspond to the magmatites of the shoshonite series and correspond to a large extent to the riftogenic and intraplate regimes. The isotopic composition of lead in dyke rocks is characterized by a linear trend and indicates mixed mantle-crustal sources of the initial substance. Lamprophyres have the least radiogenic lead, and the most radiogenic lead is characteristic of the granodiorite apophyses of the Auerbach intrusion. Lead isotopic compositions of igneous rocks and ores form a single trend on Pb-Pb diagrams, which reflects the genetic relationship of mineralization with deep magmas. The “mantle” lead in the dykes probably reflects the nature of their parent magmas, while the “crustal” lead is considered as the result of assimilation of the frame substance when the melts reach the level of the upper crust. As a result of isotopic studies of K-Ar and Sm-Nd isotopic systems, data on the Carboniferous age of post-ore basite dykes were obtained for the first time – 346 ± 4 Ma (⁴⁰Ar/³⁹Ar method) and coincidental estimate, within error, of 340 ± 35 Ma (Sm-Nd method). Rather high positive εNd values (from 4.4 to 5.3) indicate the mantle source of their magmatic melts. The petrogeochemical features of the rocks of the late dyke series, as well as their Early Carboniferous age (346–340 Ma), suggest a post-collisional mode of their introduction.